Comment by est

2 months ago

I really appreciate the engineering effort went into this "JavaScript interpreter"

https://github.com/yt-dlp/yt-dlp/blob/2025.09.23/yt_dlp/jsin...

22 comments

est

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sirbranedamuj  2 months ago

This is the buried lede in this announcement for me - I had no idea they were already going to such lengths. It's really impressive!

Aurornis  2 months ago

This is perfect for the problem they were solving. Really cool that they took it this far to avoid adding further overhead.

XnoiVeX  2 months ago

It's a subset of Javascript. HN discussion here https://news.ycombinator.com/item?id=32794081

  • extraduder_ire  2 months ago

    This description reminds me of program language wars excerpt I saw somewhere years ago about how c was obviously superior to Pascal, because with enough preprocessor macros you can compile any Pascal program in c. Followed by some hideous and humorous examples.

  • ASalazarMX  2 months ago

    This is amazing, an old school but effective approach in this modern age. I was afraid they were going to embed a browser.

codedokode  2 months ago

I decided just to look at the code for a moment and discovered ChainMap in Python.

  • ddtaylor  2 months ago

    This is excellent for some of my usages. I want to have my AI agents "fork" their context in some ways, this could be useful for that instead of juggling a tree of dictionaries.

  • bjackman  2 months ago

    Ha, that's cool. I have implemented a crappy and probably broken version of this type before. Next time I won't have to!

LordShredda  2 months ago

I'm on mobile, this seems like an actual js interpreter that only does objects and arithmetic. Impressive that it went that far

supriyo-biswas  2 months ago

Heh, now I wonder how much JavaScript it actually interprets and given that it’s < 1000 lines, whether it could be used towards an introductory course in compilers.

  • kccqzy  2 months ago

    Obviously not. An introductory course would introduce concepts like lexers, parsers, AST, etc, instead of working on strings.

    Here are lines 431 through 433:

        if expr.startswith('new '):
        obj = expr[4:]
            if obj.startswith('Date('):

  • Too  2 months ago

    There’s a famous presentation by David Beazley where he implements a WASM interpreter in Python in under an hour. Highly recommended.

jokoon  2 months ago

Wait I thought they were running an entire browser engine

  • ddtaylor  2 months ago

    Over time they probably will require that. I believe YT still allows most of these things because of "legacy" apps, which they have been killing off bit by bit. I'm not sure if anyone is cataloging the oldest supported app, but most things like using YT from a slightly older game console don't work anymore.

    Basically any publicly known method that can sip video content with doing the least work and authentication will be a common point of attack for this.

jollyllama  2 months ago

I wonder how long until it gets split off into its own project. For the time being, it could do with a lot more documentation. At least they've got some tests for it!

  • CaptainOfCoit  2 months ago

    > I wonder how long until it gets split off into its own project

    The submission is literally about them moving away from it in favor of Deno, so I think "never" probably gets pretty close.

  • zahlman  2 months ago

    Aside from the fact that the point of the announcement is that they're dropping it entirely, this "interpreter" is a hack that definitely is nowhere near capable of interpreting arbitrary JS. For example, the only use of `new` it handles is for Date objects, which it does by balancing parens to deduce the arguments for the call, then treating the entire group of arguments as a string and applying regexes to that.